Skin-Resident Memory T Cells: Pathogenesis and Implication for the Treatment of Psoriasis

Tissue-resident memory T cells (TRM) stay in the peripheral tissues for long periods of time, do not recirculate, and provide the first line of adaptive immune response in the residing tissues. Although TRM originate from circulating T cells, TRM are physiologically distinct from circulating T cells with the expression of tissue-residency markers, such as CD69 and CD103, and the characteristic profile of transcription factors. Besides defense against pathogens, the functional skew of skin TRM is indicated in chronic skin inflammatory diseases. In psoriasis, IL-17A-producing CD8+ TRM are regarded as one of the pathogenic populations in skin. Although no licensed drugs that directly and specifically inhibit the activity of skin TRM are available to date, psoriatic skin TRM are affected in the current treatments of psoriasis. Targeting skin TRM or using TRM as a potential index for disease severity can be an attractive strategy in psoriasis.


Introduction
Once the immune system encounters antigens, memory T cells are generated from the naïve T cells and facilitate a prompt response to the re-exposure of the same antigens. Two populations of memory T cells have been defined from human blood circulation: effector memory T cells (T EM ) and central memory T cells (T CM ) [1]. T EM are also dominant in peripheral non-lymphoid tissues and T CM have an affinity for secondary lymphoid organs [2,3]. Furthermore, research on murine infectious disease models has revealed that a subpopulation of T EM found in peripheral tissues remain in the same tissues for long periods without recirculation after cure of infection [4][5][6]. These findings led to the establishment of the new population of memory T cells, tissue-resident memory T cells (T RM ).
T RM are superior to their circulating memory counterparts in their ability to provide the local adaptive cellular defense [7][8][9][10][11]. They can respond to the local antigen re-exposure without the recruitment of circulating T cells to the tissue [12]. In addition, recent studies suggest T RM also contribute to systemic immune responses upon subsequent exposure to specific antigens by proliferating and baring circulating populations, such as T CM and T EM [13,14].
The existence and functional activities of T RM were initially investigated in barrier tissues, such as the gut [6,15], skin [4,5,12,16,17], respiratory tract [18,19], and reproductive tract [20,21], in the context of local defense against pathogens in infectious diseases. However, their roles are now recognized in various conditions, including cancer immunity, tissue-specific autoimmune diseases, and chronic inflammatory diseases both in barrier and non-barrier tissues [22].
Skin T RM are among the intensively studied T RM populations not only in murine models but also in humans. The human skin contains an estimate of 20 billion T cells, doubling those in the circulation [23], and over half of these T cells show the T RM phenotype [24]. Besides infectious diseases, the involvement of skin T RM has been reported in allergic contact hypersensitivity [25]; fixed drug eruption [26]; cutaneous malignancies, including malignant melanoma [27,28] and cutaneous T-cell lymphoma [24,29]; and chronic inflammatory diseases, such as vitiligo, alopecia, and psoriasis [30,31].
In this review, we provide an overview of the general characteristics of T RM . Then, narrowing our focus to skin T RM in humans, we summarize the involvement of skin T RM in cutaneous disorders, especially psoriasis. We also mention the possibility of engaging T RM as a disease index and treatment target in psoriasis. Since CD8 + T RM are the bestcharacterized population, we focus on CD8 + T RM and describe this population as T RM in this review unless otherwise mentioned.

The Characteristics of T RM
T cells in the neonatal murine skin are predominant with dendritic epidermal T cells (DETCS) with restricted antigenic specificity [32], and neonatal human skin holds only a few T cells [24]. Thus, T RM are assumed to develop from circulating T cells according to repeated antigen exposure. In the local inflammation caused by specific antigens, the robustly expanded effector T cells emerge in the circulation and the affected tissues, and both T CM and T RM are assumed to arise from a part of these effector T cells [25,33].
The general characteristics of T RM across the tissues include the loss of migration and the gain of retention. The development and maintenance of these characteristics in T RM are driven by complex factors, such as cytokine and chemokine receptors, the other cell-surface molecules being responsible for tissue homing and retention, and transcription factors ( Figure 1).
Skin TRM are among the intensively studied TRM populations not only in murine models but also in humans. The human skin contains an estimate of 20 billion T cells, doubling those in the circulation [23], and over half of these T cells show the TRM phenotype [24]. Besides infectious diseases, the involvement of skin TRM has been reported in allergic contact hypersensitivity [25]; fixed drug eruption [26]; cutaneous malignancies, including malignant melanoma [27,28] and cutaneous T-cell lymphoma [24,29]; and chronic inflammatory diseases, such as vitiligo, alopecia, and psoriasis [30,31].
In this review, we provide an overview of the general characteristics of TRM. Then, narrowing our focus to skin TRM in humans, we summarize the involvement of skin TRM in cutaneous disorders, especially psoriasis. We also mention the possibility of engaging TRM as a disease index and treatment target in psoriasis. Since CD8 + TRM are the best-characterized population, we focus on CD8 + TRM and describe this population as TRM in this review unless otherwise mentioned.

The Characteristics of TRM
T cells in the neonatal murine skin are predominant with dendritic epidermal T cells (DETCS) with restricted antigenic specificity [32], and neonatal human skin holds only a few T cells [24]. Thus, TRM are assumed to develop from circulating T cells according to repeated antigen exposure. In the local inflammation caused by specific antigens, the robustly expanded effector T cells emerge in the circulation and the affected tissues, and both TCM and TRM are assumed to arise from a part of these effector T cells [25,33].
The general characteristics of TRM across the tissues include the loss of migration and the gain of retention. The development and maintenance of these characteristics in TRM are driven by complex factors, such as cytokine and chemokine receptors, the other cell-surface molecules being responsible for tissue homing and retention, and transcription factors ( Figure 1).

Cell Surface Molecules
While homing molecules including chemokine receptors are diverse depending on the target peripheral tissues, the molecules related to tissue retention seem to be shared among various tissues. In general, TRM lack the expression of the secondary lymphoid homing molecules CC-chemokine receptor 7 (CCR7) and L-selectin, which are expressed on TCM and naïve T cells [1]. The tissue retention molecules CD69 and CD103 (αE integrin)

Cell Surface Molecules
While homing molecules including chemokine receptors are diverse depending on the target peripheral tissues, the molecules related to tissue retention seem to be shared among various tissues. In general, T RM lack the expression of the secondary lymphoid homing molecules CC-chemokine receptor 7 (CCR7) and L-selectin, which are expressed on T CM and naïve T cells [1]. The tissue retention molecules CD69 and CD103 (αE integrin) are widely recognized as the markers for T RM . CD103 is a ligand of E-cadherin that is expressed on epithelial cells [34], and CD69 interferes with sphingosine-1-phosphate (S1P) receptor-1, which allows the cells to exit from peripheral tissues by sensing the density of S1P [35]. CD69 also reportedly regulates the uptake of L-tryptophan and the intracellular quantity of L-tryptophan-derived activator of the aryl hydrocarbon receptor (AhR) [36], which is reportedly involved in the persistence of T RM [32]. These functions would explain at least partially the importance of these molecules in tissue retention. However, their expression varies, possibly depending on the tissues and the causes of T RM development. T RM lacking CD103 expression have been described in some peripheral tissues and secondary lymphoid organs [37,38] and CD103 + T RM can be found in the dermis and adult central nervous system where E-cadherin is absent, implying that binding to E-cadherin is not required for the persistence of T RM in peripheral tissues [24,39]. Although CD69 is expressed on the majority of T RM in various peripheral tissues, T RM negative for CD69 expression are also noted [33]. We thus have to take into account that these two molecules are not able to cover T RM universally.

Transcription Factors
Transcriptional regulation is also presumably common among T RM in various tissues. For instance, the expression of AhR is increased in skin T RM as compared with naïve T cells and splenic T cells, possibly favoring the maintenance of skin T RM [32]. Rapamycin inhibits the formation of T RM in the intestinal and vaginal mucosa, highlighting a positive link of mammalian target of rapamycin and the downstream transcription factors with the formation of T RM [40]. The maintenance of lung T RM may be related to Notch signaling, including the upregulation of the downstream transcription factor RBPJ [41]. The augmented uptake of exogenous lipids accompanied by the upregulation of fatty acid binding proteins (FABPs) 4 and 5 is one of the characteristic processes involved in the generation and maintenance of skin T RM [42]. Hypoxia-inducible factor-1α, which is a transcription factor in the downstream of FABP5 signaling, reportedly promotes the residency and anti-tumor function of tumor-infiltrating T cells in the murine malignancy model [43]. The downregulation of T-box transcription factors T-bet and EOMES [44] and the upregulation of Blimp-1, Hobit [45], and Runx3 [46,47] have also been reported to be involved in the differentiation and/or maintenance of T RM .

Skin-Homing Molecules
In addition to the shared characteristics of various T RM , skin T RM are shown to have their own homing molecules. As one of skin's homing molecules, cutaneous lymphocyteassociated antigen (CLA) binds to E-selectin and P-selectin and allows the cells to migrate into skin [23]. The chemokine receptors CCR4, CCR8, CCR10, CXCR3, and CXCR6 are also regarded as important skin-homing and/or retention molecules for at least some skin T cells [16,[48][49][50][51][52].

Fate Decision of T RM
How the fate of T RM differentiation is decided remains an unsolved question. T RM reportedly derive from circulating T cells lacking high expression of the killer cell lectin-like receptor subfamily G member 1 (KLRG1), which is regarded as a terminal differentiation marker [16,47]. Another report demonstrates that the effector T cells with enriched expression of T RM -associated genes, such as Itgae (CD103), Itga1 (CD49a), Cd101, Ahr, and Fabp5, already exist as memory precursor cells and preferentially differentiate into T RM [53], suggesting that the fate of T RM is at least partially decided in the early stage of adoptive immune memory formation. On the other hand, the time-course single-cell RNA-sequencing analysis in a murine model with lymphocytic-choriomeningitis-virus infection revealed that the transcriptional characteristics of T RM can be detected from gut-infiltrating T cells at the earliest 4 days after infection, and the characteristics are distinct from those found in splenic T cells [54], implying that the T RM differentiation program is initiated after the cells enter the specific peripheral tissues. Further elucidation of the T RM differentiation mechanism will require further research.

Human Skin T RM
In general, human T RM and murine T RM share core transcriptional, phenotypic, and functional profiles, including the almost global expression of CD69 and dominant CD103 expression in CD8 fractions [45,[55][56][57]. In patients with cutaneous T-cell lymphoma (CTCL), the treatment with alemtuzumab, which depletes circulating T cells and spares the T RM , does not result in serious infection [58], implying the role of skin T RM in protective immunity. The T RM phenotype of the malignant cells in CTCL is related to the clinical manifestation of well-demarcated lesions, suggesting that the sessile property of T RM also exists in humans [24]. In vitro experiments suggest skin T RM maintain the production of IL-17A and IFN-γ in reaction with pathogen challenges through aging [59]. Using transcriptomic and functional data, human T RM are found to abolish their senescent phenotype and survive for over 10 years in specific circumstances [46], replicating the longevity of T RM in humans.
However, T RM in humans are presumably more diverse and widely distributed. For instance, CD4 + T RM are found in both the epidermis and dermis in humans, although murine skin CD4 + T RM are predominantly found in dermis [17,24,60,61]. T RM are also found in secondary lymphoid organs, such as the spleen, lymph nodes, and tonsils in humans [55,56].
The factors that may cause the difference between human skin T RM properties and those observed in laboratory mice may include the following: (1) the thick epidermis with abundant niche for T RM [24,62]; (2) the low density of hair follicles that express cytokines important for T RM migration and survival, including IL-7 and IL-15 [63,64]; (3) the frequent exposure to foreign antigens; (4) the small population of γδT cells with the lack of DETC in the human epidermis [65] (however, we do not know whether the recently identified αβγδT cell population in fetal skin can replace DETC) [66]. The longer survival period of human T RM compared to murine life span [46] may also cause difficulty in adapting the findings in murine models to human biology.
The involvement of skin T RM is highlighted in chronic inflammatory disorders and cutaneous malignancies. In the lesional skin of alopecia areata, T RM with the ability to produce granzyme B are dominant and related to disease prognosis, implying their involvement in the pathogenesis [67]. Intraepidermal IFN-γ-producing T RM are enriched in the cured sites of fixed drug eruption [26], suggesting the contribution of this fraction to the reproducible property. In patients with atopic dermatitis, cutaneous T RM with the production of IL-4 and IL-13 are also indicated to be involved in the disease pathogenesis [68]. Dermal T RM are increased with the production potential of perforin, granzyme B, and IFN-γ in vitiligo [30,69], which are presumably specific for melanocyte antigens. In malignant melanoma, skin T RM provide protection against tumor regrowth and are involved in vitiligo formation, suggestive of their specific reactivity against melanoma antigens [70]. Better understanding of cutaneous T RM will pave the way for novel management and treatment of skin diseases.
The methodologies for evaluating skin T RM are summarized in Table 1. In the translational research field, one of the most popular methods for analyzing T RM is fluorescenceactivated cell sorting (FACS) analysis. However, conducting this method from biopsied skin specimens is not practical in the daily clinical settings considering the burden for both patients and clinicians. Immunohistochemistry (IHC) and/or immunofluorescence (IF) for T RM -related molecules, such as CD3, CD8, CD69, and CD103, on the residual biopsy specimens carried out for diagnosis is probably more feasible to date. To establish noninvasive methods for predicting the activities of skin T RM , such as analyzing tape-stripped or surface-swabbed samples, will require further research.

Skin T RM in the Pathogenesis of Psoriasis
Psoriasis, hereafter referred to as plaque psoriasis, is an immune-mediated chronic inflammatory skin disorder characterized by well-demarcated persistent scaly indurated erythematous plaques. The contributions of environment [71], hereditary predisposition [72], and autoantigens [73] are implied to be involved in disease development. Circulating T cells were previously regarded as responsible for the lesion formation in psoriasis. However, the inhibition of E-selectin, which is required for T-cell migration from the blood stream to skin, was noted to be ineffective [74]. Another blocking strategy of T-cell migration by the biologics targeting CD11a also did not show dramatic efficacy [75]. However, in a humanized murine model where psoriatic nonlesional skin specimens are grafted to immunodeficient mice [76], the healthy-appearing nonlesional skin grafts spontaneously develop psoriatic disease, suggesting that the cells residing in the nonlesional skin are sufficient for the development of psoriatic disease. These results have led to the theory that T RM may play a crucial role in the pathogenesis of psoriasis.
The fate of skin T RM is affected by the skin microenvironment, and in psoriasis, this is also the case. Several skin-constituting factors have been reported to support the development and persistence of IL-17A-producing T RM in psoriasis. Keratinocytes in disease-naïve sites of psoriasis upregulate the expression of chemokines, such as CCL20 upon stimulation by skin commensal fungi [77]. Since CCL20 is a ligand for CCR6, which is a signature molecule of IL-17A-producing T cells, the activated keratinocytes in the disease-naïve sites of psoriasis are to recruit IL-17A-producing T cells to the disease-naïve sites, leading to the accumulation of IL-17A-producing T RM [77]. In turn, IL-17A from T RM stimulates keratinocytes to express CCL20, further accelerating the recruitment of CCR6 + cells [78]. In the resolved skin, the continuous production of IL-23 and IL-15 from Langerhans cells presumably support the maintenance of IL-17A-producing T RM in the epidermis [79]. The reduced repertoire of IL-17A-producing T cells in the resolved skin, which has been observed in different psoriatic patients, implies the existence of common antigens that drive the accumulation of psoriatic T RM [80]. Several potential autoantigens have been reported in psoriasis ( Figure 2). For example, cationic antimicrobial peptide LL-37 produced by various cells including keratinocytes binds self-DNA and triggers the activation of plasmacytoid dendritic cells (pDC) and TNF/iNOS-producing dendritic cells (TIP-DC) [81,82]. A disintegrin-like and metalloprotease domain containing thrombospondin type 1 motif-like 5 (ADAMTSL5) in complex with HLA-C*06:02 on the surface of melanocytes confers epidermal CD8 + T-cell response [83]. Neo-lipid antigens generated by phospholipase A2 group 4D (PLA2G4D) from mast cells and keratinocytes trigger the CD1a-reactive T cells to produce IL-17A and IL-22 [84]. Keratin 17, a human epidermal keratin that shares a sequential homology with streptococcal M protein, is recognized by HLA-Cw*0602-restricted IFN-γ-producing CD8 + T cells [85,86]. Taken together, these results suggest the synchronizing roles of the skin microenvironment in the development and persistence of pathogenic cutaneous T RM .
In the lesional skin of patients with psoriasis, T RM consist of both CD4 and CD8 fractions, which synchronize the elevated immune response by the increased expression of inflammatory cytokines, such as IL-17A, IL-22, and IFN-γ [62,80,87,88]. While IL-17Aproducing CD4 + T RM also exist in healthy skin, the enrichment of CD8 + T RM producing IL-17A in the epidermis is one of the characteristics of psoriasis [87,88]. In disease-naïve skin that has never experienced disease formation, IL-17A production is augmented by T RM [77], and the increase in IL-17A-producing CD8 + T RM at the dispense of IFN-γ-producing T RM occurs according to disease duration [88].
IFN-γ-producing T RM are also dominant in the epidermis and express the complex of CD49a-CD29, also known as very late antigen 1 (VLA-1) or α1β1 integrin [76]. CD49a + T RM are involved in the pathogenesis of psoriasis. The number of epidermal CD8 + CD49a + T RM correlates with the severity of the disease [89], and an experimental blockade of CD49a in mice transplanted with psoriatic skin reduces the disease formation [76]. However, since the blockade of whole CD8 + T cells almost completely prevents disease development in the similar psoriatic skin-engrafted murine model [90], CD49a + T RM with IFN-γ production are not likely the key population for disease development, while the CD8 + T cell population likely includes a critical fraction for disease pathogenesis. In fact, CD8 + T RM without the expression of CD49a are defined as an IL-17A-producing T RM subset [30]. In the lesional skin of patients with psoriasis, TRM consist of both CD4 and CD8 fractions, which synchronize the elevated immune response by the increased expression of inflammatory cytokines, such as IL-17A, IL-22, and IFN-γ [62,80,87,88]. While IL-17A-producing CD4 + TRM also exist in healthy skin, the enrichment of CD8 + TRM producing IL-17A in the epidermis is one of the characteristics of psoriasis [87,88]. In disease-naïve skin that has never experienced disease formation, IL-17A production is augmented by TRM [77], and the increase in IL-17A-producing CD8 + TRM at the dispense of IFN-γ-producing TRM occurs according to disease duration [88].
IFN-γ-producing TRM are also dominant in the epidermis and express the complex of CD49a-CD29, also known as very late antigen 1 (VLA-1) or α1β1 integrin [76]. CD49a + TRM are involved in the pathogenesis of psoriasis. The number of epidermal CD8 + CD49a + TRM correlates with the severity of the disease [89], and an experimental blockade of CD49a in mice transplanted with psoriatic skin reduces the disease formation [76]. However, since the blockade of whole CD8 + T cells almost completely prevents disease development in the similar psoriatic skin-engrafted murine model [90], CD49a + TRM with IFN-γ production are not likely the key population for disease development, while the CD8 + T cell population likely includes a critical fraction for disease pathogenesis. In fact, CD8 + TRM without the expression of CD49a are defined as an IL-17A-producing TRM subset [30].
Successful treatment with an IL-17A-targeting biologics results in a decreased number of IL-17A-producing TRM in resolved skin, but the frequency of these cells is not altered within the remaining T cells [91]. Another study on residual psoriasis after the use of biologics revealed a decrease in keratinocyte proliferation. However, the percentage of IL-17A-producing CD103 + TRM was not significantly reduced after the treatments [92]. Similarly, a new normal in the persistence of IL-17A-producing TRM with CCR6 and IL-23R expression in the resolved skin has been established [62,80]. IL-17A-producing CD8 + TRM Successful treatment with an IL-17A-targeting biologics results in a decreased number of IL-17A-producing T RM in resolved skin, but the frequency of these cells is not altered within the remaining T cells [91]. Another study on residual psoriasis after the use of biologics revealed a decrease in keratinocyte proliferation. However, the percentage of IL-17A-producing CD103 + T RM was not significantly reduced after the treatments [92]. Similarly, a new normal in the persistence of IL-17A-producing T RM with CCR6 and IL-23R expression in the resolved skin has been established [62,80]. IL-17A-producing CD8 + T RM and IL-22-producing CD4 + T RM remain in the psoriatic epidermis for as long as six years after starting the successful TNF-α-targeting treatment [62]. Taken together, these findings highlight the essential standing point of IL-17A-producing T RM as one of the pathogenic populations of skin T RM in psoriasis.

Targeting Skin T RM in the Management of Psoriasis
Regardless of the persistence of this population by various treatments in psoriasis, many of the current and upcoming therapeutics in clinical practice presumably exert an indirect influence on cutaneous IL-17A-producing T RM . Since the remission period after successful treatments inversely correlates with the relative IL-17 signaling of the resolved skin compared to IL-10 and IFN-γ signaling [93], the relative reduction, if not elimination, of IL-17A-producing T RM may be of help in controlling psoriatic disease activity.
Biologics targeting the IL-17 pathway reportedly reduce IL-17 signaling and the amount of T cells in the lesion [94]. Furthermore, the biologics targeting IL-23 decrease this fraction from the lesion more strongly compared to those targeting IL-17A [95]. Ultraviolet irradiation leads to the diminishment of IL-17A-producing T cells in skin [96], and this T-cell fraction includes T RM . Topical vitamin D analogues and corticosteroids reportedly reduce the lesional IL-17A-producing T RM , possibly including pathogenic T RM [97,98]. Retinoic acid prevents Th17 differentiation and possibly promotes the properties of regulatory T cells [99,100]. As the oral phosphodiesterase 4 inhibitor (PDE4i) diminishes the pro-inflammatory cytokine production from circulating T cells [101], the function of both topical and systemic PDE4i could be revisited from the perspective of skin T RM . An AhR agonist modulates the Th17 property of T cells, and the efficacy of its topical form possibly affects IL-17A-producing T cells in skin, including T RM [102].
Proof-of-concept approaches that directly and exclusively target pathogenic populations of T RM should be subjected to further studies. The candidate strategies might include the inhibition of the pathways involved in IL-15 signaling to perturb the survival of pathogenic T RM and the blockade of the pathways processing fatty acids to suppress the lipid metabolism of pathogenic T RM . Targeting the transcription factors specified for differentiation and maintenance of pathogenic T RM is also an attractive strategy. However, although the risk of targeting these populations of T RM is unknown, it may cause the loss of local immune memory against pathogens in the skin. Since the characteristic cell surface molecules and transcription factors found in T RM properties can be overlapped with the sessile properties of other cell types, such as innate lymphoid cells and B cells [103,104], the strategies targeting T RM might also affect the other tissue-sessile immunity. Specific treatment targets for psoriatic dysfunctional T RM , excluding the other T RM and skin-resident immune cells, would be ideal.

Conclusions
Extensive studies with rigorous methodologies have broadened our knowledge on T RM in general and those residing in the skin in particular ( Table 1). The involvement of skin T RM in the pathogenesis of skin diseases is also being elucidated. Several key points are highlighted below: • T RM originate from circulating T cells, do not recirculate, and provide the first line of adaptive cellular defense in the residing tissues.

•
The functional skew of skin T RM is indicated in chronic skin inflammatory diseases.

•
In psoriasis, IL-17-A-producing CD8 + T RM may be among the pathogenic populations in the skin. • Pathogenic populations of skin T RM can be targeted in the current and future treatments of psoriasis. Skin T RM can also serve as a potential index of the disease.
Further studies on T RM will advance the management of not only psoriasis but other diseases in which this subset of T cells plays a role. Table 1. Several major findings related to methodologies used in research on humans.

Key Findings Major Methodologies
A role of skin T RM in protective immunity in humans FACS [58] Skin T RM with the potential of producing cytokines are infiltrated in the lesion of patients with GVHD FC, single-cell TCR sequencing, and IF [46] Cells residing in nonlesional skin are sufficient, and the recruitment of circulating cells is not necessary for the development of psoriatic disease Transplantation, FC, quantitative RT-PCR, and IHC [76] CD8 + T RM producing IL-17A in the epidermis is one of the characteristics in psoriasis FC and IHC [87] The increase in IL-17A-producing CD8 + T RM during the distribution of IFN-γ-producing T RM occurs according to psoriasis duration FC and IF [88] The successful treatment with IL-17A-targeting biologics results in a decreased number of IL-17A-producing CD8 + T RM in resolved psoriatic skin, but the frequency of these cells is not altered FC, IHC, and IF [91] IL-17A-producing CD8 + T RM and IL-22-producing CD4 + T RM remain in the psoriatic epidermis for as long as six years after starting the successful TNF-α-targeting treatment Conflicts of Interest: R.W. received lecture fees from the companies Abbvie, Eli Lilly, Janssen Pharmaceuticals, Kyowa Kirin, Maruho, and Novartis, and a research grant from the companies Maruho and Janssen Pharmaceuticals. The funders have no role in the writing or in the decision to publish this manuscript.